Drift tracking feedback for communication channels

ABSTRACT

A communication channel includes a first component having a transmitter coupled to a normal signal source, and a second component having a receiver coupled to a normal signal destination. A communication link couples the first and second components. Calibration logic provides for setting an operation value for a parameter of the communication channel, such as by executing an exhaustive calibration sequence at initialization of the link. A tracking circuit, including a monitoring function, tracks drift in the parameter by monitoring a feedback signal that has a characteristic that correlates with drift in the communication channel, and updates, or indicates the need for updating of, the operation value of the parameter in response to the monitoring function.

RELATED APPLICATION DATA

This application is a continuation of pending application Ser. No.12/619,076, entitled Drift Tracking Feedback for Communication Channelsfiled 16 Nov. 2009 (allowed), which application is a continuation ofapplication Ser. No. 11/744,006, entitled Drift Tracking Feedback forCommunication Channels filed 3 May 2007 (now U.S. Pat. No. 7,640,448);which is a continuation of application Ser. No. 11/560,031, entitledDrift Tracking Feedback for Communication Channels filed 15 Nov. 2006(now U.S. Pat. No. 7,526,664); which is a continuation of applicationSer. No. 11/243,032 entitled Drift Tracking Feedback for CommunicationChannels filed 4 Oct. 2005 (now U.S. Pat. No. 7,159,136); which is acontinuation of application Ser. No. 10/802,634 entitled Drift TrackingFeedback for Communication Channels filed 17 Mar. 2004, (now U.S. Pat.No. 6,961,862); which prior applications are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the calibration of communicationchannel parameters in systems, including mesochronous systems, in whichtwo (or more) components communicate via an interconnection link; and tothe calibration needed to account for drift of conditions related tosuch parameters during operation of the communication channels.

2. Description of Related Art

In high-speed communication channels, including channels which areoperated in a mesochronous manner, a transmitter and receiver are eachlocked to a specific phase relative to some global reference clock. Thephase values of the transmitter and receiver which minimize thebit-error-rate of the communication channel may be determined in anumber of ways. However, once these phase values are set, offsets due tosystem conditions (such as temperature effects on the propagation timeof the wires in the link) will cause the chosen phase values to becomenon-optimal. Other parameters of the communication channel also driftdue to changing system conditions.

It is therefore desirable to provide techniques to detect and compensatefor the drift in parameters of a communication channel due to changingsystem conditions like these.

SUMMARY OF THE INVENTION

The present invention provides a system and method for setting aparameter of a communication channel, which allows for optimizing theparameter while efficiently tracking drift of properties of the channel.A communication channel includes a first component having a transmittercoupled to a normal signal source, and a second component having areceiver coupled to a normal signal destination. A communication linkcouples the first and second components. The communication systemprovides for setting an operation value for the parameter, which forexample can include executing an exhaustive calibration sequence atinitialization of the channel. The method and system of the presentinvention provides for tracking drift in the parameter by monitoring asignal on another channel that is characterized by drift relative to areference, such as a reference clock, in amounts that correlate withdrift in the parameter of communication channel relative to thereference, and to update, or indicate the need for updating, theoperation value of the parameter in response to the monitoring function.The signal being monitored is provided in various ways according toembodiments of the invention, including by signals on channels thatemulate the channel being monitored, by signals on another channelwithin a single communication bus, by signals on a channel that loopsback to a component on the channel that generates it, or by signals thatotherwise exhibit characteristics that correlate, whether causally ornot, to the parameter being tracked on the channel.

In embodiments of the invention, the parameter is updated by executing acalibration sequence on the communication link. In other embodiments,the updating includes adjusting the operation value of the parameter asa function of the drift determined by monitoring the signal.

According to embodiments of the present invention, the parameter of thecommunication channel is the phase of one of the receive and transmitclocks for an input/output driver, where the phase establishes a sampletiming point for a receiver, or a drive timing point for a transmitter.The monitoring function operates by comparing the phase of the signalwith the phase of a reference clock. Because the signal being monitoreddrifts in phase by amounts that correlate with the drift in phase of thesample timing point or drive timing point of communication channel beingadjusted, the relative phase changes can be relied upon to indicate theneed for updating the operation value of the parameter.

The signal being monitored in order to track drift in variousembodiments comprises a data transmission signal, for example, onanother communication channel. The monitoring step, in some embodiments,includes detecting transitions in the data transmission, and comparingthe detected transitions with transitions in the reference clock. Inother embodiments, the monitored signal comprises a control signal usedfor controlling communications in the communication channel, such as adata strobe between the components using the communication channel. Inyet other embodiments, the monitored signal comprises a signal takenfrom outside the communication channel that nonetheless can be reliedupon for emulating drift in the parameter.

The invention is also embodied by an apparatus coupled with a driver fora communication channel. The apparatus includes memory to store theoperation value of the parameter, logic to monitor a signal which has acharacteristic that correlates with the parameter of the communicationchannel to track drift in the characteristic, and logic to indicate theneed to update, or to update directly, the operation value based on thedrift in the monitored signal.

An embodiment of the present invention comprises an interface for acommunication bus. The interface includes a plurality of input/outputdrivers coupled with respective bidirectional links in the communicationbus. The input/output drivers include a transmitter responsive to atransmit clock, which drives a signal on a corresponding link in thecommunication bus. Also, a receiver is included in the input/outputdriver, that is responsive to a receive clock. The receiver samples asignal on the corresponding link. A transmit clock generator and areceive clock generator are coupled respectively to the transmitter andthe receiver, and to a reference clock. At least one of the transmitclock generator and the receive clock generator is adjustable inresponse to a phase control signal. The interface also includes a drifttracking circuit. The drift tracking circuit is coupled to a linkcarrying a monitored signal that is characterized by drift thatcorrelates with drift in the communication bus. The drift trackingcircuit includes a tracking clock generator that is coupled to thereference clock and generates a tracking clock. A phase detector isincluded in the drift tracking circuit, to track changes in phasebetween the tracking clock and transitions in the monitored signal.Logic in the drift tracking circuit monitors the changes in phase, andissues an indication that the phase control signal be updated to accountfor the changes in phase.

The receive clock generator and the transmit clock generator in someembodiments of the interface comprise a phase locked loop, coupled tothe reference clock, which produces a set of output clocks that are somemultiple of the input reference frequency. A mixer is coupled with thephase locked loop, and is responsive to the phase control signal togenerate the transmit or receive clock, having an adjustable phase asindicated by the phase control signal. Also, memory coupled with theclock generator stores the phase control signal.

In yet other embodiments of the interface, calibration logic is includedto determine the operation value of the phase control signal. Thecalibration logic is responsive to the indication from the drifttracking circuit, to re-calibrate the operation value of the phasecontrol signal. Also, the calibration logic may be executed to initiallyset the operation value of the phase control signal, for example atstart up of the interface.

Accordingly, the present invention provides for drift tracking based ondata collected during normal operation of a communication channel,without relying upon periodic calibration sequences that remove thecommunication channel from its mission function. Thus, the presentinvention tracks drift of parameters of the communication channel,including mesochronous channels, while preserving overall throughput andlatency characteristics of the link. The present invention isparticularly suited to high speed communication channels, includingchip-to-chip channels with links across traces on printed circuitboards, or other board level media, operating with system clocks over100 MegaHertz, including clocks with frequencies in the range of 400MegaHertz, and higher.

Other aspects and advantages of the present invention can be seen onreview of the drawings, the detailed description and the claims, whichfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of two components interconnected by acommunication channel, with drift tracking according to an embodiment ofthe present invention.

FIG. 2 is a simplified diagram of two components interconnected by acommunication channel, with drift tracking according to anotherembodiment of the present invention.

FIG. 3 illustrates an embodiment of an interface to a communicationchannel, including input/output drivers and a drift tracking circuitaccording to the present invention.

FIG. 4 is a flow chart of operation of calibration logic used to set theoperation value of the parameter of the communication channel in someembodiments of the present invention.

FIG. 5 illustrates an embodiment of an interface to a communicationchannel between a memory controller and a memory, such as a DRAM,including input/output drivers and a drift tracking circuit according tothe present invention with data strobe monitoring.

FIG. 6 illustrates an embodiment of an interface to a communicationchannel between first and second chips, including input/output driversand a drift tracking circuit according to the present inventionincluding a loop back channel carrying a monitored signal.

DETAILED DESCRIPTION

A detailed description of embodiments of the present invention isprovided with reference to the Figures.

FIG. 1 shows two components 10, 11 connected with an interconnectionmedium, referred to as link 12. Component 10 has a transmitter circuit13 which drives symbols (bits) on link 12 in response to timing eventson the internal CLKT signal 14. This series of bits forms signal DATAT.Component 11 has a receiver circuit 15 which samples symbols (bits) onlink 12 in response to timing events on the internal CLKR signal 16. TheDATAT forms signal DATAR at the opposite end of the link, which issampled by the receiver. The transmit clock CLKT is generated by atransmit clock generator 40. The receive clock CLKR is generated by areceive clock generator 49 that is responsive to a phase control signalstored in a phase calibration register 48. A communication channelbetween the components 10 and 11 comprises the transmitter circuit 13,the link 12 and the receiver circuit 15. Delay block 50 in the link 12represents delay encountered in the communication channel, which maydrift during operation due to changing operational and environmentalconditions in the components and in the link. For successful datatransmission, the relative phases of the transmit clock CLKT and thereceive clock CLKR must account for the delay represented by block 50.As the delay represented by block 50 drifts, the relative phases of thetransmit clock CLKT and the receive clock CLKR must also be adjusted inorder to maintain successful data transmission.

A tracking circuit 45 on component 11 is coupled to a monitored signalMS on line 46. Line 46 represents a channel, which has a characteristic,such as the drift in phase, which correlates with drift in delay 50 inthe communication channel across link 12. The tracking circuit 45generates an indication on line 47 used to control the value in thephase register 48, which is coupled to the receive clock generator 49.The indication is generated when the drift in the monitored signal online 46 reaches a predetermined level in some embodiments. In oneembodiment, the tracking circuit 45 generates the indication on line 47on each phase change of the monitored signal, and adjusts the phasecontrol signal in phase register 48 in response to a function of thephase changes indicated by the drift tracking circuit 45. In anotheralternative, the indication on line 47 is provided to calibrationresources, such as special purpose logic, a host processor executingcalibration routines, or other resources, which execute a calibrationsequence for the link 12 to update the phase control signal in phaseregister 48.

In the embodiment of FIG. 1, the tracking circuit is on the receive sideof the channel. In alternative embodiments, the tracking circuit isimplemented on the transmit side of the channel, or on both the transmitand the receive sides of the channel. FIG. 2 illustrates an embodimentwith drift tracking for a parameter of a transmitter in a communicationchannel between first and second components. Thus, FIG. 2 shows twocomponents 10, 11 connected with an interconnection medium, referred toas link 12. Component 10 has a transmitter circuit 13 which drivessymbols (bits) on link 12 in response to timing events on the internalCLKT signal 14. This series of bits forms signal DATAT. Component 11 hasa receiver circuit 15 which samples symbols (bits) on link 12 inresponse to timing events on the internal CLKR signal 16. This series ofbits forms signal DATAR. The transmit clock CLKT is generated by atransmit clock generator 40. The receive clock CLKR is generated by areceive clock generator 49. A communication channel between thecomponents 10 and 11 comprises the transmitter circuit 13, the link 12and the receiver circuit 15. Delay block 50 in the link 12 representsdelay encountered in the communication channel, which may drift duringoperation due to changing operational and environmental conditions inthe components and in the link. For successful data transmission, therelative timing for the communication channel is determined by the phaseof the transmit clock CLKT and the phase of the receive clock CLKR. Thedifference in phase between the two clocks is absorbed by the delayrepresented by delay block 50.

A tracking circuit 45 on component 10 is coupled to a monitored signalMS on line 46. Line 46 represents a channel, which has a characteristicsuch as the drift in phase, that correlates with drift of a parameter inthe communication channel across link 12. The tracking circuit 45generates an indication on line 47 used to control the transmit timing,which is coupled to the transmitter circuit 13. The indication isgenerated when the drift in the monitored signal on line 46 reaches apredetermined level in some embodiments.

FIG. 3 illustrates a system including a first integrated circuit 200 anda second integrated circuit 201 which communicate via a communicationbus, generally 202. In the system of FIG. 3, a clock generator 250 isincluded, which provides a reference clock to the master CTM on thefirst integrated circuit 200. The first integrated circuit 200, in thisexample provides the reference clock from the master CFM to the secondintegrated circuit 201.

The communication bus includes a plurality of communication channelsthat comprises a transmitter, a communication link and a receiver. Forexample, the communication bus includes a plurality of bi-directionaldata links represented by line 203, a plurality of bi-directionaladdress links represented by line 204, and a plurality of bi-directionalcontrol links represented by line 205. In other embodiments, thecommunication bus comprises multidrop links, uni-directional links, orcombinations of multidrop, uni-directional and bi-directional links.

The first integrated circuit 200 and the second integrated circuit 201include interface circuits supporting the communication bus. Forsimplicity, an interface circuit for one of the bi-directional datalinks 203 on the communication bus is shown.

The interface includes an input/output driver 210. The input/outputdriver includes a transmitter 211 and a receiver 212. The transmitter211 accepts input from a normal path used for the mission function ofthe first integrated circuit 200 and drives the corresponding data onthe communication link 203. A transmit clock CLKT is coupled to thetransmitter 211 and sets a drive timing point. A receiver (not shown) onthe second integrated circuit 201 samples the data from the line 203 inresponse to a corresponding receive clock. The drive timing point on thetransmitter 211 is set by calibration logic 213, using for example acalibration routine as described below, so that the receiver on thesecond integrated circuit 201 is able to capture correct data.

Likewise, a receive clock CLKR is coupled to the receiver 212, and setsa sample timing point. The sample timing point is set by the calibrationlogic 213, in cooperation with a transmitter (not shown) on the secondintegrated circuit 201 in exemplary embodiments.

The transmit clock and the receive clock are produced by clock generatorcircuits. The clock generator circuits include a phase locked loop 214,which produces a set of reference clocks, and supplies those clocks tomixers 215, 216, 217 in the interface. The mixer 215 is used forgenerating the transmit clock in response to a mixer control signalstored in register 218. The mixer 216 is used for generating the receiveclock in response to a mixer control signal stored in register 219. Inthe illustrated embodiment, registers 218 and 219 are managed by thecalibration logic 213. In alternative embodiments, the registers couldbe managed by higher-layer circuits which implement the equivalent ofthe calibration logic 213 in software.

The mixer 217 is used for generating a tracking clock TR-CLK, which isalso based on the reference clock CTM so that tracking relative to thetracking clock TR-CLK is, thereby, tracking relative to the referenceclock CTM in some embodiments. The tracking clock TR-CLK is supplied toa phase detector 228, which also receives as input a monitored signalfrom another channel 204 on the communication bus. In this example, themonitored signal is supplied on line 229 from the input to the receiver227. The phase detector 228 includes a transition detector, so thattransitions in the monitored signal and transitions in the trackingclock are utilized for detecting shifts in phase of the monitoredsignal. The output of the phase detector 228 is supplied to a counter230, which counts up and counts down to follow shifts in phase. Theoutput of the counter 230 is used as a phase control signal for themixer 217. Monitor logic 231 is coupled to the counter 230, and issuesindications to the calibration logic 213, in response to changes in thecounter value.

Input/output drivers, like driver 210, are coupled to each of the linksin the communication bus. The tracking circuit including the mixer 217,the phase detector 228, and the counter 230, can be repeated for eachinput/output driver, or shared amongst sets of input/output drivers onthe communication bus, or shared among all the links on the bus,depending on level of accuracy needed, the environment of operation forthe particular implementation, and other trade-offs.

The calibration logic 213 may include additional registers, such asregisters storing edge values for use during calibration routines asdescribed below, in addition to registers storing the operation valuefor each of the clocks. The registers 218 and 219 are loaded by thecalibration logic 213 depending on the mode of operation.

Accordingly, the system shown in FIG. 3 performs timing calibrationduring normal communication between integrated circuits in the system.The calibration of read and write clocks in the integrated circuits maybe executed without affecting the normal communication between theintegrated circuits. In the embodiment shown, the system includes aclock-data recovery loop which tracks drift by locking itself to anarriving monitored signal, such as read data sent from anotherintegrated circuit device. Based on this clock-data recovery loop,timing correction of the sample clock used to receive data, and thedrive timing clock used to transmit data, may be derived.

According to the present invention, long-term variances in system timingparameters, such as time-of-flight of a data signal, can be detected.Based on this detection, various timing calibration adjustment processescan be initiated. For example, the present invention can be designed sothat changes in phase in the tracking circuit of more than a threshold,for example changes of 10 percent, from the state recorded during aninitial calibration, result in generation of an indication to thecalibration logic that a re-calibration sequence should be executed.Alternatively, when the phase of the tracking clock is shifted by athreshold amount, a compensating adjustment could be made to the phasecontrol values for the input/output driver being monitored.

FIG. 4 shows calibration steps executed by calibration logic in oneexemplary embodiment of the invention, which is suitable for use incombination with the drift tracking circuit described above, in someembodiments to set up initial operation values of the parameters, toperiodically update the operation values in response to indications fromthe drift tracking circuits, or to otherwise supplement the drifttracking circuit.

(Step 301) On initialization, or upon an indication from a drifttracking circuit that a drift measurement is due, suspend transmit andreceive operations that are in progress.(Step 302) Change the sample point of the receiver component from thenormal operation value (used for normal operations) to a value used forcalibration operations, such as an edge value representing an edge of atiming window for successful reception. It may be necessary to impose asettling delay at this step to allow the new drive point to becomestable.(Step 303) Decouple the transmitter from the normal data path.(Step 304) Create and transmit a pattern onto the “link.” Note that thedrive point of the transmitter is fixed relative to the reference clockof the system and is not adjusted.(Step 305) The pattern set is received in the receive component usingthe calibration value of the sample point.(Step 306) The received pattern set is compared to an expected patternset produced in the receive component. The two pattern sets will eithermatch or not match. As a result of this comparison (and possibly otherprevious comparisons) a pass or fail determination will be made.(Step 307) Adjust the sample point operation value in the receivercomponent as a result of the pass or fail determination.(Step 308) Repeat steps 302-307 to locate edge values in phase for thesample point outside of which the calibration pattern is notsuccessfully received.(Step 309) Change the sample point of the receiver to a new operationvalue (used for normal operations). It may be necessary to impose asettling delay at this step to allow the new drive point to becomestable.(Step 310) Reselect the “normal path” input of the transmitter.(Step 311) Resume normal transmit and receive operations.

FIG. 5 illustrates an embodiment of the present invention for acommunication channel 203 between a first chip 600 which may be a memorycontroller as labeled in the drawing, and a second chip 601 which may bea memory like a double data rate DDR dynamic random access memory DRAMas labeled in the drawing, that provides a DQS data strobe signal duringread operations. The reference numerals in FIG. 5 are the same as thosefor components of FIG. 3, with the exception of items relating to thedata strobe channel 602. Other components of FIG. 5 are not describedagain, as they correspond with like numbered components of FIG. 3. Inone advantageous embodiment of FIG. 5, the monitored signal for thedrift tracking circuit comprises the DQS data strobe control signalproduced by the DDR DRAM during read operations (or other data strobesignals or other control signals as appropriate for a particularimplementation), and received in data strobe I/O 610 by receiver 603 andprovided to read control logic (not shown). The monitor logic 231 isconfigured to monitor the received read strobe, the transmitted readstrobe, or both. When it is configured to track drift from bothtransmitted read strobes and received read strobes, the logic includesmode control to switch between the two modes. In these embodiments, themonitor function can use shared resources to track drift in the localtransmitter, and drift in the combination of the transmission line andthe remote transmitter. Of course the shared resource embodiment can beapplied to other types of monitored signals.

Also, the data strobe I/O has calibrated transmit and receive clocks,generated by mixers 604 and 606 respectively, in response to operationvalue parameters stored in registers 605 and 607.

FIG. 6 illustrates an embodiment of the present invention for acommunication channel 203 between a first chip 200 and a second chip201, where the monitored signal is provided on a loop back link 650. Thereference numerals in FIG. 6 are the same as those for components ofFIG. 3, with the exception of the loop back channel including the link650, the monitored signal transmitter 651, the pattern source 653, andthe monitored signal receiver 652. Other components of FIG. 6 are notdescribed again, as they correspond with like numbered components ofFIG. 3. In the embodiment shown, a pattern source 653 is coupled to thetransmitter 651. The transmitter transmits the pattern, and the receiverreceives the looped back pattern. The monitor logic 231 tracks changesin the phase of the looped back pattern. The pattern source 653 can bereplaced by any available signal on the chip 200, such as the CTMreference clock, that has transition density or other characteristicsmaking it suitable for use in tracking drift.

The link 650 is implemented in some implementations using the sametransmission line design as used for the links 203 of the communicationchannels being adjusted. For example, the chip-to-chip bus isimplemented using printed circuit board traces optimized for frequency,length, amplitude of the signal and other parameters. The loop backchannel 650 uses the same or similar printed circuit board tracetechnology, and has a length of for example one-half the average lengthof the traces forming the channels in the bus, so that the round triptime of flight is close to the time of flight for signals on the bus.

In one advantageous embodiment of FIG. 6, the loop back channelcomprises a transmitter 651 and a link 650 which emulate behavior of thetransmitter 211 and link 203 of the communication channel beingadjusted. Thus, the looped back signal being monitored from the receiver652 tracks the behavior of a loop back transmitter 651 on the same chipas the transmitter 211 being adjusted. In other embodiments describedabove, the transmitter driving the monitored signal is located on aremote chip. Also, in embodiments like that of FIG. 6, the step 301shown in FIG. 3 may not be necessary during period calibrationoperations, if the periodic calibration operations are performed usingthe loop back channel.

In the embodiments described above, a communication channel is providedhaving an additional phase mixer, which generates a tracking clock,coupled to a phase detector. The phase detector circuit compares thealignment between the tracking clock and a monitored signal. Themonitored signal can consist of either a looped back clock signal, asignal from the memory chip on the opposite end of the communicationchannel, one of the data bits transmitted on the channel, one of thecontrol bits or address bits transmitted on the channel, or any othersignal which correlates with the drift on the communication channel. Thetracking circuit, which consists basically of a clock-data recoverycontrol block for systems tracking timing parameters, processes theoutput of the phase detector and affects adjustments in a counter plusregister circuit which controls the phase mixer that generates thetracking clock. A monitor circuit periodically compares the currentvalue stored in the register, with previous values, and generatesoutputs based on that comparison. This output can be used simply toindicate that a re-calibration is required, or for real-time adjustmentof the operation value of the parameter being monitored.

According to the present invention, communication systems are providedthat track drift, while conserving resources of the communicationchannels being calibrated. The invention reduces the hardware andstorage requirements needed for calibration, reduces the impact onbandwidth and throughput across the communication channel, and maintainsaccuracy of the operation values of the parameters being calibratedwhile using less of the resources of the communication channel. Also,the techniques of the present invention improve overall latencycharacteristics of the communication channel in changing operatingconditions.

A method for calibrating a communication channel, including a firstcomponent, a second component, and a communication link coupling thefirst and second components, the communication channel having aparameter with an operation value is described, comprising:

setting the operation value of the parameter of the communicationchannel in one of the first and second components;

distributing a reference clock signal to the first and secondcomponents;

monitoring, using monitor circuits in one of the first and secondcomponents, a signal from a medium outside the communication channel,the signal having a characteristic which drifts relative to thereference clock signal at said monitor circuits by amounts thatcorrelate with drift of the parameter of the communication channel, totrack the amounts of drift in the characteristic; and

in response to the monitoring, updating the operation value based on theamounts of drift in the characteristic of the signal.

In embodiments of such method, said updating includes executing acalibration sequence.

In embodiments of such method, said updating includes adjusting theoperation value as a function of the amounts of drift.

In embodiments of such method, monitoring includes comparing a phase ofthe reference clock signal with a phase of said signal.

In embodiments of such method, said signal comprises a data signaltransmission in another communication channel between the first andsecond components, and said monitoring includes recovering a clock fromthe data transmission, and comparing said clock with the reference clocksignal.

In embodiments of such method, said signal comprises a control signaltransmission in another communication channel between the first andsecond components.

In embodiments of such method, the parameter comprises a drive timingpoint for a transmitter on the first component.

In embodiments of such method, the monitoring circuits are on the firstcomponent, and the parameter comprises a drive timing point for atransmitter on the first component.

In embodiments of such method, the monitoring circuits are on the firstcomponent, and the parameter comprises a sample timing point for areceiver on the second component.

In embodiments of such method, the monitoring circuits are on the firstcomponent, and the monitored signal is transmitted by the secondcomponent.

In embodiments of such method, the monitoring circuits are on the firstcomponent, and the monitored signal is transmitted by the firstcomponent.

In embodiments of such method, the monitoring circuits are configuredfor monitoring first signals transmitted by the first component andsecond signals by the second component, and including logic to selectthe monitored signal from one of the first signals and second signals

An apparatus coupled with a communication channel, the channel includinga first component, a second component, and a communication link couplingthe first and second components, the communication channel having aparameter with an operation value is described, comprising:

memory in one of the first and second components to store the operationvalue of the parameter of the communication channel;

a reference clock;

monitor logic in one of the first and second components to monitor asignal outside the communication channel, that has a characteristicwhich drifts relative to the reference clock by amounts that correlatewith drift of the parameter of the communication channel relative to thereference clock, to track the amounts of drift in the characteristic;and

logic, responsive to the monitoring, to update the operation value basedon the amounts of drift.

In embodiments of such apparatus, said logic to update includes logic toexecute a calibration sequence.

In embodiments of such apparatus, said logic to update includes logic toadjust the operation value as a function of the drift.

In embodiments of such apparatus, said monitor logic includes a phasecomparator to compare a phase of the reference clock with a phase of thesignal.

In embodiments of such apparatus, said signal comprises a data signaltransmission in another communication channel, and said logic to monitorincludes a data/clock recovery circuit to recover a clock from the datatransmission in the other communication channel, and to compare saidclock with a reference clock signal.

In embodiments of such apparatus, said signal comprises a control signaltransmission in the communication channel.

In embodiments of such apparatus, the parameter comprises a drive timingpoint for the transmitter on the first component.

In embodiments of such apparatus, the parameter comprises a sampletiming point for the receiver on the second component.

In embodiments of such apparatus, logic is included to execute acalibration sequence to determine the operation value, and store theoperation value in said memory.

In embodiments of such apparatus, said communication channel comprisesone of a plurality of communication channels of a communication bus, andsaid signal is carried on a communication channel that comprises one ofplurality of communication channels in the communication bus.

In embodiments of such apparatus, said communication channel comprisesone of a plurality of communication channels of a communication bus, andsaid signal is carried on a communication channel that comprises anotherone of plurality of communication channels in the communication bus, andthe signal comprises a data strobe.

In embodiments of such apparatus, said communication channel comprisesone of a plurality of communication channels of a communication bus, andthe signal is carried on a communication channel that comprises anotherone of plurality of communication channels in the communication bus, andthe signal comprises a timing reference signal generated by one of thefirst and second components on the communication channel.

In embodiments of such apparatus, the monitoring logic is on the firstcomponent, and the monitored signal is transmitted by the secondcomponent.

In embodiments of such apparatus, the monitoring logic is on the firstcomponent, and the monitored signal is transmitted by the firstcomponent.

In embodiments of such apparatus, the monitoring logic is configured formonitoring first signals transmitted by the first component and secondsignals by the second component, and including logic to select themonitored signal from one of the first signals and second signals.

An interface for a communication bus is described, comprising:

a plurality of input/output drivers coupled with respectivebi-directional links in the communication bus, the input/output driversincluding

-   -   a transmitter, responsive to a transmit clock, which drives a        signal on a corresponding link in the communication bus;    -   a receiver, responsive to a receive clock, which samples a        signal on the corresponding link;    -   a transmit clock generator coupled to the transmitter and to a        reference clock having a frequency over 100 MegaHertz, which        generates the transmit clock;    -   a receive clock generator coupled to the receiver and to the        reference clock, which generates the receive clock, wherein at        least one of the transmit clock generator and the receive clock        generator is adjustable in response to a phase control signal;        and

a drift tracking circuit, coupled to a link carrying a monitored signalthat is characterized by drift relative to the reference clock thatcorrelates with drift of parameters in the communication bus relative tothe reference clock, the drift tracking circuit including

-   -   a phase detector to track changes in phase between the reference        clock and the monitored signal, and

logic to monitor the changes in phase, and issue an indication that thephase control signal, for said at least one of the transmit clockgenerator and the receive clock generator, be updated to account for thechanges in phase.

In embodiments of such interface, said link carrying a monitored signalcomprises one of the links in the communication bus and the monitoredsignal comprises a data strobe on the link.

In embodiments of such interface, said link carrying a monitored signalcomprises one of the links in the communication bus and the monitoredsignal comprises a timing reference signal generated by a device coupledto the link.

In embodiments of such interface, said link carrying a monitored signalcomprises one of the bi-directional links in the communication bus, andthe monitored signal comprises a data signal on the bi-directional link.

In embodiments of such interface, said link carrying a monitored signalcomprises one of the bi-directional links in the communication bus, andthe monitored signal comprises a control signal on the bi-directionallink.

In embodiments of such interface, said link carrying a monitored signalcomprises a link emulating a bi-directional link in the communicationbus.

In embodiments of such interface, said receive clock generator comprisesa phase-locked loop, coupled to the reference clock, and producing a setof frequency divided output clocks, a mixer, responsive to the phasecontrol signal and the set of frequency divided output clocks, togenerate the receive clock, and memory storing a value of the phasecontrol signal.

In embodiments of such interface, said transmit clock generatorcomprises a phase-locked loop, coupled to the reference clock, andproducing a set of output clocks, a mixer, responsive to the phasecontrol signal and the set of output clocks, to generate the transmitclock, and memory storing a value of the phase control signal.

In embodiments of such interface, calibration logic is included todetermine a value of the phase control signal, and wherein saidcalibration logic is responsive to said indication to re-calibrate thevalue of the phase control signal

In embodiments of such interface, logic responsive to said indication toadjust a value of said phase control signal based on said detectedchanges in phase.

In embodiments of such interface, the phase detector detects the phasebetween the reference clock and the monitored signal in response to aphase difference between one of the receive and transmit clocks, and themonitored signal.

In embodiments of such interface, a tracking clock generator is includedcoupled to the phase detector and to the reference clock which generatesa tracking clock, and wherein the phase detector detects the phasebetween the reference clock and the monitored signal in response to aphase difference between one of the tracking clock and the monitoredsignal.

In embodiments of such interface, memory is included to store a transmitphase control signal for the transmit clock generator and to store areceive phase control signal for the receive clock generator.

In embodiments of such interface, the monitored signal is transmitted bya remote component.

In embodiments of such interface, the monitored signal is transmitted atthe interface.

In embodiments of such interface, the drift tracking circuit isconfigured for monitoring first signals transmitted by the interface andsecond signals transmitted by a remote component, and including logic toselect the monitored signal from one of the first signals and secondsignals.

A method for calibrating a communication channel, including a firstcomponent, a second component, and a communication link coupling thefirst and second components, the communication channel having aparameter with an operation value determined by calibration isdescribed, comprising:

setting the operation value of the parameter of the communicationchannel in one of the first and second components;

supplying from the first component, a monitor signal on a channel whichloops back to the first component;

monitoring, using monitor circuits in the first components, a signalfrom a medium outside the communication channel, the signal having acharacteristic which drifts relative to the reference clock signal atsaid monitor circuits by amounts that correlate with drift of theparameter of the communication channel, to track the amounts of drift inthe characteristic; and

in response to the monitoring, updating the operation value based on theamounts of drift in the characteristic of the signal.

A method for calibrating a communication channel, including a firstcomponent, a second component, and a communication link coupling thefirst and second components, the communication channel having a drivetiming point with an operation value determined by calibration isdescribed, comprising:

setting the operation value of the drive timing point of thecommunication channel;

monitoring a signal from a medium outside the communication channel, thesignal having a characteristic which drifts by amounts that correlatewith drift of the drive timing point of the communication channel, totrack the amounts of drift in the characteristic; and

in response to the monitoring, updating the operation value based on theamounts of drift in the characteristic of the signal.

A method for calibrating a communication channel, including a firstcomponent, a second component, and a communication link coupling thefirst and second components, the communication channel having a drivetiming point with an operation value determined by calibration isdescribed, comprising:

setting the operation values of the drive timing point and sample timingpoint of the communication channel;

monitoring a signal from a medium outside the communication channel, thesignal having a characteristic which drifts by amounts that correlatewith drift of the drive timing point and sample timing point of thecommunication channel, to track the amounts of drift in thecharacteristic; and

in response to the monitoring, updating the operation values of thedrive timing point and sample timing point based on the amounts of driftin the characteristic of the signal.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims.

What is claimed is:
 1. A method, comprising: setting the operation valueof a parameter of a communication channel; monitoring a signal from amedium outside the communication channel, the signal having acharacteristic which drifts relative to a reference by amounts thatcorrelate with drift of the parameter of the communication channel, totrack the amounts of drift in the characteristic; and in response to themonitoring, updating the operation value based on the amounts of driftin the characteristic of the signal.